Thanks heartless nut for strange outburst (7)
I believe the answer is:
tantrum
'outburst' is the definition.
(I've seen this before)
'thanks heartless nut for strange' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'heartless' says to hollow out the word (remove centre letters).
'for' is a charade indicator (letters next to each other).
'strange' becomes 'rum' (similar in meaning).
'nut' with its middle taken out is 'nt'.
'ta'+'nt'+'rum'='TANTRUM'
(Other definitions for tantrum that I've seen before include "Frenzy" , "Childish display of temper" , "Fit - pet" , "Childish fit of anger" , "Uncontrolled outburst of anger" .)